1. Field of the Invention
The present invention relates to a multi-tube type X-ray CT apparatus comprising a plurality of pairs of X-ray tubes and X-ray detectors, and a filter.
2. Description of the Related Art
An X-ray CT apparatus (X-ray computed tomography apparatus) reconstructs a tomogram on the basis of a plurality of projection data sets acquired from a plurality of directions by rotation of a pair of an X-ray tube and a detector. A multi-tube type X-ray CT apparatus comprises a plurality of pairs. A multi-tube type X-ray CT apparatus is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 2004-73406. The apparatus disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2004-73406 includes an X-ray tube for medical treatment and an X-ray tube for data acquisition.
As shown in FIG. 4, in a multi-tube type X-ray CT apparatus, it is a problem that scattered radiation originating from X-rays generated by an X-ray tube 122 (121) of one pair is detected by an X-ray detector 131 (132) of the other pair.
The adverse effect of this detection is not small because scattered radiation generated by portions β1 and β2 of the surface of a subject P directly reach the X-ray detectors 131 and 132 without being attenuated by the subject P.
Note that filters (to be also referred to as wedge filters) are arranged between the X-ray tubes 121 and 122 and the subject P. As shown in FIG. 5A, each of the filters F1 and F2 has a sectional structure which has a thin middle portion and thick peripheral portions and whose thickness changes arcuately in accordance with changes in spread angle θ. More specifically, the filters F1 and F2 are designed such that the intensities of X-rays transmitted through the filters F1 and F2 and a cylindrical homogeneous phantom become almost constant, as shown in FIG. 5B.